During Chapter 2, it is possible to grab Wheatley as he comes out of the wall in Test Chambers 2 and 7. He cannot be harmed by lasers and will not be destroyed by the Emancipation Grill, but disappears after taking the elevator to the next Test Chamber.

When Portal 2's audio files are tampered with or deleted, in the place of a broken audio clip, the game will instead play a file called "error.wav". This audio clip is of Dr. Kleiner from Half-Life 2, taken from a scene in which Dr. Kleiner is beginning to teleport Alyx but the cord gets unplugged, saying "Oh fiddlesticks, what now?"

During the development of Portal 2, playtesters found GLaDOS to be far too mean and vindictive. The developers concluded that a shift in personality was necessary, to better reflect the change in the power dynamic between Chell and GLaDOS. As a result, some of her dialogue was changed to make GLaDOS more amicable and passive-aggressive.

In an earlier version of the co-op plot, GLaDOS sends Atlas and P-Body out to find "artifacts" of the long-dead Aperture workers (coffee cups and such) to make them more human. Unused audio files from this period of development can still be found within the games files, revealing that in one of the rooms a comic strip is found, presumably Garfield from how it is described.

There is an unused Turret hidden in the game files called the Hover Turret. It has no programming other than to follow the player with its laser sight. It also has no model, so it shows up as a default ERROR model.

After the Authoring Tools update, the Adhesion Gel (which allowed Chell to defy gravity) was replaced with the Reflection Gel, a gel that reflects lasers. It also reflects the turrets' laser sights, but this is just a cosmetic feature and doesn't affect their actual "vision".

There was originally going to be a song featured in the game called "GLaDOS' Song". It was written and performed by Ellen McLain, the voice actor for GLaDOS. She wrote the song for use as an Easter Egg in the game but it was ultimately never used.

The song was actually recorded after the game's release, and was performed by both Ellen and her husband, John Lowrie, the voice of the Sniper from Team Fortress 2. It was later modified by Harry Callaghan to sound closer to GLaDOS' synthesized voice.

There are unused lines for Cave Johnson found within the games '.nut' files. The lines would've been used for a scene where the player finds an AI cube with Cave Johnson's conscious programmed inside it. He would meet GLaDOS and reveal that she indeed used to be Caroline and that he "volunteered" her to be a test subject to see if his conscious could be loaded into a machine. He would then continuously ask the player to kill him.

From 2014 to 2016, The Game Awards featured a "Best Fan Creation" category. The 2014 winner was "Twitch Plays Pokémon" (a social experiment featuring a crowdsourced attempt to play through various Pokémon games), and the 2015 winner was "Portal Stories: Mel" (a modification of Portal 2). During 2016, however, two of the nominees for the category were removed prior to the show's airing: AM2R (an unofficial remake of Metroid II: Return of Samus) and Pokémon Uranium (a fan-made video game based on the Pokémon series). During a stream prior to the event, host Geoff Keighley explained that this was due to the fangames not being legally cleared by Nintendo, who owned the rights to the games they were based on. Additionally, "Best Fan Creation" would end up being the only category to not have its winner (Enderal: The Shards of Order, a conversion mod for The Elder Scrolls V: Skyrim) announced during the ceremony itself.

In certain scenes in Portal 2, the player is required to quickly place a portal to get out of a dangerous situation. According to developer Dave Saunders, during playtesting, it was noticed that lots of players were dying during these segments because they placed the wrong color portals, so the developers specifically changed it so that clicking any button will result in the correct colored portal.